A history of the pyrolytic application of powder coatings to the surface of a glass sheet is set forth hereinbelow.
U.S. Pat. No. 3,081,200, in column 3, line 13, shows that the patentee has sprayed both solutions and powder in order to form pyrolytic metal oxide coatings on a heated glass sheet. The patent examples, however, are directed to solutions of coating materials and no specific instructions are contained in the patent with respect to how one successfully accomplishes the application of powders to form a pyrolytic metal oxide film on a glass sheet. Also, the patent contains no instructions with respect to the manner of making up a powder which may be successfully applied to a glass sheet to form a pyrolytic coating film thereon.
German Pat. Nos. 2,529,076; 2,529,077; and 2,529,079 deal with the development of a metal oxide film on a glass sheet.
In German Pat. No. 076, metal salt particles are charged prior to application to a glass sheet which has been preheated to a temperature in the range from 500.degree.-650.degree. C. The patent indicates that heat decomposable, organic based metal salts may be used and that a preheated carrier gas may be used for transporting the particles to the glass sheet. The patent teaches that the particles should be charged and distributed in a 90.degree. downwardly directed attitude toward the glass sheet from a gas distributor which extends transverse to the direction of travel of the glass sheet passing therebelow.
German Pat. No. '077 discloses a process for forming a metal oxide film on a glass sheet by pyrolytic techniques. The starting material is a powder metal compound which is carried by an inert carrier gas into a cyclone where the smallest particles of the compound are separated from the coarser particles, subsequently carried along by the carrier gas flow to the surface of the glass sheet to be coated. The coarser particles are collected then partly evaporated under an inert atmosphere by heating. Thereafter, the resulting metal compound vapors are mixed with the carrier gas which carries the finer particles to the surface to be coated. Thus the patent teaches the simultaneous application of both relatively fine heat decomposable, organic based metal salts and vaporized material of the same nature to the glass surface which is to have a metal oxide film developed thereon.
German Pat. No. '079 discloses a method in which relatively large particles of a heat decomposable material are dropped on a glass sheet to develop a film thereon.
U.S. Pat. No. 3,852,098 discloses a process in which a glass sheet is coated with a metal containing film by heating the glass and contacting the hot glass with a gaseous mixture. The patent does not deal with the application of powder materials directly to the surface of the glass to develop a metal oxide coating thereon. Rather, the process disclosed is one which uses as the coating media a gaseous mixture containing 40-100% by volume of the vapors of a reactive metal compound. The mixture is heated by the glass to a sufficient temperature, causing the metal vapor compound to react, thereby depositing the coating film.
U.S. Pat. No. 4,182,783 is a patent directed to a vapor deposition process for achieving a metallic oxide film on a glass sheet. The method discloses fluidizing particulate solid coating reactants by first establishing a fluidized bed of dispersed particulate solid coating reactants and thereafter drawing a volume of fluidized gas and suspended particulate solid coating reactants to a vaporizer. An additional volume of gas is mixed therewith and then all of the particulate solid coating reactants are vaporized in the reactant/gas mixture. This reactant/gas mixture is then directed into contact with a hot substrate to be coated to deposit a film thereon.
U.S. Pat. No. 4,188,199 discloses a process for forming a metal compound coating on the face of a continuously longitudinally moving glass ribbon. The ribbon face is contacted while at an elevated temperature with a fluid medium containing a substance which undergoes chemical reaction or decomposition to form the metal compound on the surface of the moving glass ribbon.
U.S. Pat. No. 4,230,271 discloses an apparatus for evenly and uniformly depositing particulate material suspended in a gas on at least one surface of a glass sheet. In particular, the glass sheet may be a strip of glass and the product placed thereon a metal salt.
The principal of the process of the present invention is to provide a process for developing a coating film on the surface of a sheet of glass, the glass sheet being heated to a temperature in the range of 510.degree.-625.degree. C. This process is characterized by engaging the surface of the heated glass sheet with a special heat decomposable powder spray composition.